The present invention relates generally to compositions and methods for treating set gypsum products and to methods for making set gypsum-containing materials. More particularly, the invention relates to compositions and methods for treating and making set gypsum such that the set gypsum is imparted with enhanced characteristics, including surface hardness, strength, thermal stability, paintability, abrasion resistance, and/or water erosion resistance.
Set gypsum (calcium sulfate dihydrate) is a well-known material that is used to make many types of products and that is included commonly in many types of products. Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, and optionally other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product. Mild heating can be used to drive off unreacted water to yield a dry product.
There is a continuing need to improve performance of certain characteristics, particularly surface characteristics, of the set gypsum for many products that include set gypsum as a major component. In this respect, the set gypsum surface of many conventional set gypsum-containing products is susceptible to water erosion, and to abrasion and scratching during use because of the generally poor surface characteristics of the set gypsum. It is desirable, therefore, to improve the strength and surface hardness of the set gypsum component of such products.
By way of example, set gypsum is a major component of molds made of plaster that are used in casting pottery figurines and the like using pressure casting or slip casting techniques known in the art. In such castings, wet material, for example, a mixture of water and clay is added to the mold. The plaster mold is designed to be porous so that it can absorb water that is drawn out from the clay being molded to form greenware. During the casting process, the mold becomes saturated with water and the water must be removed periodically from the mold, for example, via compressive air purging or other drying techniques, to form the desired molded product (greenware). Because the set gypsum of the plaster mold is somewhat soluble in water, the continual transfer of water into and out of the mold causes the set gypsum of the plaster mold to erode.
Many conventional plaster molds also are thermally and dimensionally unstable to some degree. Plaster molds often are subjected to sustained elevated temperatures, for example, to temperatures of 140xc2x0 F. or higher, for a time period of, for example, 30 minutes or longer. In particular, surface re-calcination of the set gypsum can also occur and has been observed. Surface re-calcination undesirably results in a loss of some strength because a portion of the set gypsum matrix is weakened. For example, surface re-calcination can cause the set gypsum component to contract and then cause cracks to form in the plaster molds. It also would be desirable to extend the use-life of plaster molds by enhancing the surface hardness, strength, and abrasion resistance of the molds.
In a similar way, it also would be desirable to improve the surface characteristics of many other products that include set gypsum as a major component. For example, conventional exterior statuary also can experience water erosion caused by exposure to moisture from any of the known sources, including condensation in the form of rain, snow and ice. In addition, exterior and interior statuary are prone to abrasions and, accordingly, improving the abrasion resistance of the set gypsum matrix of statuary is likewise desirable. Paintability, or paint adhesion to the set gypsum surface of set gypsum-containing products such as statuary, is also desirable, for example, in order to minimize the amount of paint needed to adequately cover the surface of the product, to minimize paint chipping, and the like. There further is a need to provide set gypsum-containing statuary having improved thermal and dimensional stability. Conventional statuary, when made, must be dried at relatively low temperatures, inasmuch as elevated temperatures (e.g., 100xc2x0 C. or higher) can lead to cracking and thermal contraction of the set gypsum. Statuary made of set-gypsum that is thermally and dimensionally stable could be dried at elevated temperatures, thereby enhancing the overall efficiency of the process.
Set gypsum also is included in veneer plaster, such as, for example, DIAMOND(copyright) brand interior finish plaster, commercially available from United States Gypsum Company, used to finish interior wallboards. Veneer plasters are formulated to have a relatively soft surface so as to facilitate the application thereof to a substrate. As a result, the surface hardness and strength of the veneer plaster is compromised such that the plaster is more susceptible to scratching or abrasions. This is particularly troublesome when lime is added as an ingredient to the product, as is typical to enhance the workability of the veneer compound. Accordingly, the product""s ability to resist abrasions is compromised.
It also is known to combine set gypsum with cellulosic fibers, for example, gypsum-wood fiber board such as FIBEROCK(copyright) brand composite panels, commercially available from United States Gypsum Company. During the preparation of this type of panel, calcined gypsum (e.g., alpha calcium sulfate hemihydrate) and water penetrate the cellulosic fiber pores. Upon rehydration, the set gypsum forms in situ within and about the pores of the cellulosic fibers, thereby adding strength to the product. The set gypsum-fiber composite can suffer from water erosion, abrasions, and blisters. In addition, the water required to form the set gypsum causes blistering of the fibers. As a result, such products do not bind well with paint and, therefore, exhibit poor paintability such that relatively large quantities of paint are required.
Set gypsum also is known to be used as a major component in floor underlayments. Moreover, there is an increasing demand for poured floor underlayments comprising set gypsum that include an embedded hot water heating system. However, the heat can cause some re-calcination of the set gypsum, thereby leading to thermal contraction and the formation of unsightly cracks in the floors. Set gypsum-containing floor underlayments also are vulnerable to water erosion and abrasion.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for set gypsum products that exhibit enhanced abrasion resistance, water erosion resistance, surface hardness, paintability, strength, thermal stability, or the like. There is also a need for a composition and method for treating set gypsum to satisfy any one or more of these needs.
The present invention provides a set gypsum-containing material, a composition for coating a set gypsum-containing material, a method for treating a set gypsum-containing material, and a method for making a set gypsum-containing material.
It has been found that treating set gypsum with various combinations of monobasic phosphate salts, trimetaphosphate salts and acyclic polyphosphate salts having at least three phosphate units impart to the set gypsum enhanced surface characteristics, strength, and thermal and dimensional stability. Treating set gypsum, as will be appreciated by those skilled in the art, can include post-set treatment of set gypsum. It also is possible to include trimetaphosphate salt during the formation of the set gypsum product followed by post-set treatment with one or both of the remaining types of salts, and still achieve the surprisingly enhanced surface strength, and stability characteristics of the set gypsum product. As will be appreciated, many different treatment combinations can be used while still achieving the benefits of the invention as described herein.
Thus, in one aspect, the present invention provides an aqueous composition for coating a set gypsum-containing material, which is formed from at least one member from each of the classes of phosphate salts described above and which preferably imparts to the set gypsum-containing material an improvement in one or more of its strength, surface hardness, water erosion resistance, or abrasion resistance. The aqueous composition is formed from at least one monobasic phosphate salt; at least one trimetaphosphate salt; and at least one acyclic polyphosphate salt having at least three phosphate units. In some embodiments, a water dispersible polymer or a water soluble polymer, such as a polymer latex or the like, is also used to form the composition.
The present invention also provides a method of treating a set gypsum-containing material. The method comprises applying to the set gypsum-containing material at least one member from each of at least two of the following types of inorganic phosphate salts: monobasic phosphate salts, trimetaphosphate salts, and acyclic polyphosphate salts having at least three phosphate units. In some embodiments, the set gypsum-containing material is treated further with a water dispersible polymer or a water soluble polymer, such as a polymer latex or the like. Preferably, the inorganic phosphate salts are applied in amounts sufficient to impart the set gypsum-containing material with an improvement, as compared with the untreated set gypsum-containing material, in one or more of the following characteristics: strength (e.g., flexural), surface hardness, abrasion resistance, water erosion resistance, or the like.
In another aspect, the present invention provides a method of making a set gypsum-containing material. The method comprises forming a set gypsum matrix from water, calcined gypsum, and at least one trimetaphosphate salt. The set gypsum matrix is then treated with at least one of the following types of inorganic phosphate salts: monobasic phosphate salts and acyclic polyphosphate salts having at least three phosphate units. In this aspect of the invention, post-set treatment is used to treat the set gypsum with the salts described above.
Advantageously, the compositions and methods of the invention enhance set gypsum-containing products by improving one or more desirable characteristics of the products such as, for example, surface hardness, abrasion resistance, water erosion resistance, paintability, thermal stability, such as cracking resistance, dimensional stability such as shrinkage resistance, strength and the like. The compositions and methods of the invention can be used with any of a number of set gypsum-containing products, including, by way of example and not in limitation, plaster molds for pottery casting, interior/exterior statuary, veneer plasters, gypsum-cellulosic fiber products, floor underlayments, and the like.
These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The invention may best be understood with reference to the following detailed description of the preferred embodiments.